1. Field of the Invention
The present invention relates to reticles used in patterning radiation sensitive layers, and more specifically to a reticle using attenuated phase-shifting for use in a lithography system and method incorporating multiple exposures of the scribeline region.
2. Background Information
In the semiconductor industry, there is a continuing effort to increase device density by scaling device size. State of the art devices currently comprise device features having dimensions well below 1 micron (submicron). To form these features, a photosensitive layer is formed on a substrate or device layer, and is exposed to radiation through a reticle (also referred to as a mask). The reticle typically comprises a substantially transparent base material with an opaque layer having the desired pattern formed thereon, as is well known. At the submicron level, diffraction effects become significant, resulting in exposure of portions of the photoresist layer underlying the opaque layer near the edges of features.
To minimize the effects of diffraction, phase-shifted reticles have been used in the prior art. Typically, a phase-shifted reticle has an opening in the opaque layer corresponding to the pattern to be formed. Phase-shifters, which transmit the exposing radiation and shift the phase of the radiation approximately 180.degree. relative to the openings, lie along or near the outer edges of the features. The radiation transmitted through the phase-shifter destructively interferes with radiation from the opening, thereby reducing the intensity of radiation incident on the photoresist surface underlying the opaque layer near a feature edge.
Another type of phase-shifted mask is the attenuated phase-shifted mask (APSM). The APSM replaces the opaque layer of prior art masks (which is typically a layer of chrome about 0.1.mu. thick) with a "leaky" layer which transmits a reduced or attenuated percentage of the radiation incident thereon. For example, a very thin layer of chrome (approximately 300 .ANG.) could be used as the leaky layer. A chrome layer this thin will transmit approximately 10% of the radiation incident on the reticle. Additionally, the leaky chrome layer shifts the phase of the transmitted radiation approximately 20.degree.-30.degree. compared to radiation transmitted through regions of the reticle where the leaky chrome layer is not present ("openings" in the leaky chrome layer). In order to achieve the desired 180.degree. shift, an additional transparent layer can be added to the portion of the reticle having the attenuating layer, or the openings can be phase-shifted an additional 150.degree., either by etching the reticle base or by placing a phase-shifting material in the openings. Thus, an APSM used to pattern a positive photoresist layer comprises a layer of leaky chrome covering the entire reticle base, except for the features, which are open regions (i.e. regions having no thin chrome layer) with appropriate additional phase-shifting. Since the radiation transmitted through the features is phase-shifted 180.degree. relative to radiation transmitted through the leaky chrome layer, phase-shifters are not required. Although the leaky chrome transmits less radiation than the openings, such that radiation transmitted through this region does not completely cancel the diffracted radiation from the edges of the features, there is still a sufficient cancellation to reduce the effects of diffraction so as to improve the resolution compared with non phase-shifted reticles. While the nonattenuated phase-shifters placed next to the edges of features, described above, result in more complete cancellation, in some applications the APSM may be advantageous compared with the use of the non-attenuating phase-shifters. For example, in fabricating the APSM, feature definition and phase-shifting can be performed with a single masking step, eliminating the problem of misalignment between feature and shifter. Further, the APSM can be used for certain types of features for which it is impossible or impractical for form phase-shifters.
Since the leaky chrome layer of the APSM transmits about 10% of the exposing radiation, regions away from the openings are partially exposed. In a positive photoresist, this partial exposure removes some of the photoresist from these regions, but a continuous layer remains. The photoresist layer thickness is adjusted for this exposure. For example, if the photoresist layer underneath the leaky chrome is reduced in thickness 1,000 .ANG. due to the exposure, and a 10,000 .ANG., layer of photoresist after developing is desired, an initial photoresist layer of 11,000 .ANG. will be used. In the case of a negative photoresist, the partial exposure through the attenuating layer is insufficient to harden the resist, so that portions of a negative resist exposed through the attenuating layer will be removed upon developing. The leaky chrome layer is also referred to as an "absorptive phase-shifter" and as a "halftone chrome layer." As an alternative to the leaky chrome layer, an attenuating layer having a thicker chrome layer with a sub-resolution pattern formed therein is described in co-pending U.S. patent application Ser. No. 07/952,061 filed Sep. 25, 1992 entitled "Attentuated Phase-Shifted Reticle Using Sub-Resolution Pattern," which application is assigned to the assignee of the present application, and which application is hereby incorporated by reference.
A problem occurs in APSM's used in any type of stepping system, such as a step and repeat or a step and scan system, where only a portion or "stepping field" of the wafer is exposed at one time. In these types of systems a first stepping field is stepped to and exposed, followed by stepping to the next stepping field which is similarly exposed. This is repeated until all stepping fields on the wafer have been exposed. As is well known, these stepping fields often overlap, typically in the region between the individual die, known as the scribeline region. FIG. 1 shows an example of a portion of a prior art reticle 100 used in such systems. Region 101 is the portion of the reticle having the pattern for the device die, which contains openings formed in the attenuating layer to define the active device features as described above. Region 102 surrounding region 101 is the above-described scribeline. This region comprises the unpatterned attenuating chrome layer which is typically supplied on the reticle blank. Also present in scribeline region 102 are the metrology cells 105-108. These regions contain various patterns which are used for measurements during manufacturing. For example, these metrology cells 105-108 may contain critical dimension (CD) measurement features, alignment verniers, resolution patterns, etc.
In stepping systems, the system exposes adjacent stepping fields as shown in FIG. 2, where the second exposure is denoted using the numerals of FIG. 1 with a prime (') symbol. Typically, the exposure overlaps along the scribeline region 102. As shown, the right-hand portion of the scribeline region 102 of the first exposure is overlapped by the left-hand portion of the scribeline region 102' of the second exposure. The overlap region is shown as 201 in FIG. 2. As shown, metrology cell 106 of the first exposure, shown dashed, is overlaid by the second exposure. Similarly, as the next row underneath the row shown in FIG. 2 is exposed, the top and bottom portions of the scribeline region will similarly overlap. In some systems, more than one die may be present in a stepping field; however, there is typically overlap in the scribeline region at the top, bottom, and either end of the multiple die stepping field.
As mentioned above, the attenuating layer of reticle 100 transmits approximately 10% of the incident radiation. While a single exposure does not cause any problems in the field region away from the openings as described above, the additional blanket exposure that occurs in the scribeline region results in multiple exposure of the metrology cells 105-108, causing a significant degradation of these patterns upon development. In some cases, the resist layer may be completely exposed resulting in essentially complete removal (positive resist) or hardening (negative resist). Thus, the metrology features used to control the device fabrication process cannot be successfully patterned, resulting in loss of in-line measurement, accurate overlay of successive layers at the wafer level, and control of the process. As shown in FIG. 2, all portions of scribeline region 102 will be exposed at least two times, and some portions, e.g. the corners, will be exposed up to four times. Note that in prior art, non-attenuated masks, scribeline regions 102, outside of metrocells 105-106, can be made of a thick, completely opaque layer of chrome. With the layout of metrocells 105-108 shown in FIGS. 1 and 2, this would result in each metro cell being completely blocked from multiple exposure during stepping. However, it is not practical to form the scribeline regions 102 of a thick layer of chrome on an APSM, as the region 101 would have to be masked while additional chrome is deposited and then etched from regions 102 during the fabrication of reticle 100. Such a procedure would require complex processing, and would add to the defect density of the reticle. In addition to the metrology cells 105-108, there may be other features, such as trademarks, logos, copyright or maskwork notices, etc., which are to be printed in the scribeline region 102. Finally, other processing steps or manufacturing requirements may dictate whether the scribeline region 102 is to be masked or unmasked, so that the overlapping exposure will limit the ability to control the masking of this region. For example, in some cases the manufacturer may desire to leave some or all of the scribeline region 102 masked for subsequent processing. In a positive photoresist, the overlapping exposures as described above, will cause partial to complete removal of the resist layer upon development. Even if a partial layer is left, subsequent etch processing, for example, may remove the partial layer, resulting in unwanted etching of these regions. The undesired patterns thus formed can lead to further defects in subsequent processing steps. Similarly, in some cases, the manufacturer may desire to leave the scribeline region 102 unmasked. In a negative photoresist the overlapping exposures may result in the scribeline region 102 being masked, again contrary to the manufacturer's requirement.
What is needed is an attenuated phase-shifted reticle for use in a method and system wherein multiple exposure at the edge of each stepping field occurs. The reticle and method should avoid costly and complex processing, yet allow for the patterning of features in the multiple exposure regions.